1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to an alignment film for aligning the liquid crystal (LC) in liquid crystal display (LCD).
2. Background of the Related Art
Liquid crystals (LC) are liquids consisting of anisotropic molecules. The average direction of the long molecular axis is named as director of the LC. The director distribution in the bulk LC is determined according to its anchoring on rigid substrates and is characterized by the direction of the axis of easy director orientation corresponding to the minimum LC surface energy, and an anchoring energy.
Reorientation of the director under the action of an applied external electric field form the operational basis of LC devices. The basic unit of the LC devices—displays in particular—includes an LC cell that consists of LC material provided between two rigid substrates. In order to obtain uniform brightness and a high contrast ratio within the LC display, homogeneous alignment of LC material in the cell is necessary.
Many materials are well known for their abilities to homogenously align LC materials. Polyimide, polyamide and polysilicone polymers are well known materials that may provide high quality, uniaxial, thermostable alignment of various LC mixtures. To obtain mono-domain planar alignment of LC material on the aforementioned polymers, a special mechanical surface treatment (e.g., stretching, microgroove formation, mechanical rubbing) is required. Rubbing processes can provide high quality, strong alignment of LC materials, wherein orientation is determined according to the direction of rubbing. However, the aforementioned techniques have some drawbacks. In particular, microgrooves inherently contain defects that cause random phase distortions and scatter light, thereby degrading display characteristics. Static electricity generated during the rubbing of the polymer surface is also known to generate dust and cause defects in active matrix LC displays. Moreover, it is very difficult to selectively orient selected regions of an LC surface so that each region has different orientation.
Other techniques that do not use mechanical treatment of the surface are also known. These other techniques include oblique evaporation of inorganic materials on a substrate, use of anisotropic Langmuir-Blodgett (LB) films, and anisotropic adsorption of LC molecules in a magnetic field. These techniques provide adequate homogeneous alignment but are complex and have low alignment reliability.
Another non-mechanical alignment technology includes photoalignment of liquid crystals. In photoalignment, LC material is aligned according to an anisotropic surface produced during a photochemical reaction under polarized UV light. LC material is oriented unidirectionally on the irradiated surface, and liquid crystal orientation is determined by the direction of polarization of UV light. A number of alignment film materials have been developed for photoalignment of LC materials. For example, polyvinylcinnamates (PVCN), polysiloxancinnamates (PSCN), cellulose cinnamates (CCN), coumarin-containing polyacrylates (CCP), and polyimides (PI) have been used in photoalignment technology.
The capability of photoaligning materials to align LC material is provided by appearance of the anisotropy on the surface due to irradiation of the UV light. For example, anisotropy in cinnamoyl-containing polymers is caused by a photo-crosslinking reaction. When exposed to incident light, side cinnamoyl fragments parallel to the polarization of the incident light efficiently undergo photo-crosslinking. Anisotropy in PIs is created by the photo-destruction of main PI chains. When exposed to incident light, photosensitive PI fragments parallel to the polarization of the incident light are efficiently destroyed.
The aforementioned photoaligning materials may provide adequately homogenous planar and tilted alignment of standard LC mixtures for twist-and-vertical mode LC cells (a technique used in manufacturing LC cells of display devices.
The problem that prevents wide application of photoalignment techniques in LCD manufacture, however, is a residual effect (i.e., conservation of a previous image on the display after a change of the image on the screen) introduced by conventional alignment films.
In the case of the cinnamoyl-containing materials, the modification of polymer surface by LC molecules and their reorientation in electric field induces the residual effect. Cinnamoyl-containing materials are known to provide low anchoring energy (W<10−2 erg/cm−2). The reorientation of the director on the aligning surface is essential and flexible fragments of polymer (e.g. non-crosslinked cinnamoyl-fragments) follows the director reorientaion in an electric field. Therefore, the residual effect is induced because both the easy axis and the anchoring energy of the aligning layer are altered in an electric field.
PI materials usually have no flexible side groups and their surfaces are much more rigid. Nevertheless, these photoaligning materials also possess a strong residual effect. The origin of this effect is a screening of the applied electric field due to a double charged layer near the irradiated PI surface. The double charged layer appears due to generation of electric charges in PI during UV exposure.